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Croskey,  John  Welsh,  I858- 


Anatomy  and  physiology  of  the  eye  and  iti 
appendages . 


UNIVERSITY  OF  CALIFORNIA 
SAN  FRANCISCO  LIBRARY 


Anatomy  and  Physiology 


% 


OF  THE 


YE  AND  ITS  Appendages 


BY 

JOHN  WELSH  CROSKEY,  M.  D. 

OPHTHALMIC  SURGEON 

TO  THE 

PHILADELPHIA  GENERAL  HOSPITAL 
Philadelphia,  Pa. 


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Anatomy  and  Physiology 


OF  THE 


Eye  and  its  Appendages 


BY 

JOHN  WELSH  [CROSKEY,  M.  D. 

OPHTHALMIC  SURGEON 

TO  THE 

PHILADELPHIA  GENERAL  HOSPITAL 
PhUadelphia,  Pa. 


PUBLISHED  BY 

SMITH-BDWAUDS  CO. 

129  NORTH   laTH  STREET 

PHILADELPHIA 


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PREFACE 


This  little  pamphlet  is  the  outgrowth  of  the  instruction  given 
to  students  and  nurses  at  the  Philadelphia  General  Hospital  during 
the  past  ten  years.  The  author  believes  that  if  the  anatomy  of  the 
eye  is  thoroughly  understood,  it  renders  subsequent  study  less  diffi- 
cult. 

Every  effort  has  been  made  to  simplify  the  text  so  that  the 
subject  matter  can  be  readily  understood. 

The  author's  thanks  are  due  to  his  son-in-lavsr,  Mr.  Lew^is 
Elberon  Bailey,  for  the  tw^o  most  excellent  illustrations. 


DEDICATION 


Affectionately  dedicated 

to 

my  preceptor ^  the  late  Henry  Earnest  Goodman^  M.  Z>., 

to  whom  I  owe  much  of  my  success. 


Anatomy  and  Physiology  of  the  Eye 
AND  ITS  Appendages 


JOHN    WELSH    CROSKEY,   M.  D. 

OPHTHALMIC  SURGEON 

TO  THE 

PHILADELPHIA  GENERAL  HOSPITAL 

PHILADELPHIA,  PA. 


ANATOMY, 

The  eye  is  the  organ  of  vision. 

Vision  is  the  faculty  or  sense  of  sight. 

The  Orbits  are  the  cavities  which  contain  the  eyeball,  and  are  four. 
sided,  pyramidal,  boney  cavities,  with  the  bases  in  front  and  the  apices  be- 
hind. They  are  formed  of  seven  bones:  superior  maxillary,  malar,  frontal, 
palate,  sphenoid,  ethmoid  and  lachrymal. 

The  orbital  plate  of  the  superior  maxillary,  a  portion  of  the  malar,  the 
orbital  process  of  the  palate  bone  form  the  floor  of  the  orbit.  The  orbital 
plate  of  the  frontal  and  a  small  portion  of  the  sphenoid  form  the  roof.  The 
ethmoid,  lachrymal,  sphenoid  and  the  nasal  process  of  the  superior  maxillary 
form  the  inner  wall,  while  the  outer  wall  is  formed  by  the  sphenoid  and 
malar. 

The  Floor  of  the  Orbit  is  inclined,  being  higher  on  the  nasal  side 
than  on  the  temporal  side.  It  is  thin  and  smooth  and  separates  the  orbital 
cavity  from  the  antrum  of  Highmore. 

The  Roof  of  the  Orbit,  except  at  its  margins  and  especially  the 
anterior  margin,  is  very  thin,  is  easily  penetrated  and  quite  unable  to  resist 
pressure  from  either  side.  On  its  opposite  sides  are  the  frontal  sinus  anter- 
iorly, and  the  cranial  cavity  posteriorly. 

The  anterior  margin  of  the  roof  is  thick  and  overhangs  the  orbital 
cavity.  The  inner  wall  is  extremely  thin  and  upon  its  opposite  side,  in 
front,  is  the  frontal  sinus;  below  and  behind  is  the  nasal  cavity.  The  outer 
wall  is  very  thick  at  the  rim,  thin  immediately  behind  the  rim,  then  thick 
again  on  the  sphenoidal  portion.  On  the  opposite  side  of  the  frontal  half 
of  this  wall  is  the  temporal  fossa.  At  the  posterior  portion,  the  wall  sepa- 
rates the  orbit  from  the  cranial  cavity  and  at  its  extreme  portion,  both 
cavities  are  united  through  the  sphenoidal  fissure. 


The  Foramina  of  the  Orbit  are  nine  in  number: 

1.  The  Optic  Foramen,  for  the  passage  of  the  optic  nerve  and  ophthal- 
mic artery. 

2.  The  Sphenoidal  Foramen  (fissure  Foramen  Lacerum  Anterius), 
for  the  passage  of  the  third,  the  fourth,  the  ophthalmic  division  of  the  fifth 
and  sixth  cranial  nerves,  some  filaments  from  the  cavernous  plexus  of  the 
Bjmapathetic,  the  orbital  branch  of  the  middle  miningeal  artery,  a  recurrent 
branch  from  the  lachrymal  artery  to  the  dura  mater,  and  the  ophthalmic 
vein. 

8.  The  Spheno  Maxillary  Foramen  (fissure),  for  the  passage  of 
the  superior  maxillary  nerve  and  its  branches,  the  infraorbital  artery  and 
ascending  branches  from  Meckel's  ganglion. 

4.  The  Lachrymai,  Foramen  (groove),  is  occupied  by  the  lachrymal 
or  nasal  duct. 

5.  The  Supraorbital  Foramen  (sometimes  a  groove),  for  the  passage 
of  the  supraorbital  artery,  vein  and  nerve. 

6.  The  Infraorbital  Foramen  (groove),  for  the  passage  of  the  in- 
fraorbital nerve  and  artery. 

7.  The  Malar  Foramen  (sometimes  two),  for  the  passages  of  the 
facial  nerve  and  vessels. 

8  and  9.  The  Anterior  and  Posterior  Ethmoidal  Foramina,  for 
the  passage  of  the  anterior  Ethmodial  artery  and  the  nasal  nerve. 

The  orbit  is  lined  by  periosteum  which,  after  entering  the  orbit  through 
the  optic  foramen,  is  divided  into  two  parts :  one  part  lining  the  orbit  and  the 
other  part  continuing  in  a  fibrous  sheath  known  as  Tenon's  capsule,  which 
invests  the  posterior  parts  of  the  eyeball. 

As  the  periosteum  enters  the  orbit  through  the  optic  foramen  it  forms 
a  tendinous  ring,  which  ring  gives  origin  to  the  ocular  muscles. 

The  Orbital  Periosteum  is  covered  with  a  layer  of  connective  tissue 
and  fat,  upon  which  the  eye  rests. 

In  the  roof  of  the  orbit  are  two  depressions:  the  fovea  trochlaris  at 
the  inner  angle,  and  the  fossa  lachrymalis  at  the  outer  anterior  angle. 


THE  EYE  LIDS. 

The  Eye  Lids  are  the  curtains,  movable  at  will,  that  cover  the  eyeballs 
in  front.  They  are  the  appendages  of  the  eye  and  are  composed  of  integu- 
ment, connective  tissue,  orbicularis,  palpebrarum  muscle,  tarsal  cartilage. 
Meibomian  glands,  and  conjunctiva. 

The  opening  formed  by  the  edges  of  the  lids  is  called  the  palpebral 
fissure.  This  fissure  at  the  outer  side  is  called  the  outer  canthus,,  at  the  inner 
side  the  inner  canthus.  The  outer  canthus  is  more  acute  than  the  inner,  and 
the  lids  here  lie  in  close  contact  with  the  globe ;  but  the  inner  canthus  is  pro- 
longed for  a  short  distance  inwards,  towards  the  nose,  where  the  two  lids  are 
separated  by  a  triangular  space  called  the  lachrymal  lake  or  locus  lachrymalis^ 

6 


The  lids  contain  a  semilunar  framework  of  condensed  connective  tissue 
which  is  called  the  tarsal  cartilage.  This  gives  the  lids  their  shape  and  firm- 
ness. In  the  upper  lid  it  is  oval  and  is  thickest  at  its  anterior  edge.  In  the 
lower  lid  it  is  thinner  and  narrower  and  is  of  nearly  uniform  breadth 
throughout.  The  lids  are  held  firmly  in  position  by  fibrous  tissue  internally 
to  the  tendo  oculi,  externally  to  the  malar  bone,  and  above  and  below  to 
the  margins  of  the  orbit  by  the  palpebral  ligament.  The  upper  lid  contains 
about  30  large  sebaceous  glands  called  Meibomian  glands,  and  the  lower 
lid  contains  about  20.  The  ducts  from  these  glands  open  on  the  free  borders 
of  the  posterior  lip  of  each  lid.  Their  function  is  to  secrete  a  sebum  which 
lubricates  the  edges  of  the  lids,  preventing  them  from  sticking  together  and 
also  preventing  the  overflow  of  the  lachrymal  secretions. 

The  Eyelashes  (Cilia)  are  attached  to  the  free  edges  of  the  eyelids. 
They  are  short,  thick,  curved  hairs,  arranged  in  a  double  or  triple  row  at  the 
margin  of  the  lids ;  those  of  the  upper  lid  are  more  numerous  and  longer  than 
the  lower,  and  are  curved  upwards;  those  of  the  lower  lid  are  curved  down- 
wards, by  which  means  they  do  not  interlace  on  closing  the  lids. 

The  Conjunctiva  coats  the  posterior  surface  of  each  lid  and  also  the 
anterior  surface  of  the  eyeball.  It  is  divided  into  three  portions:  the  part 
covering  the  posterior  surface  of  each  lid,  which  is  known  as  the  conjunctiva 
tarsi;  that  part  which  coats  the  anterior  surface  of  the  eyeball  called  the 
conjunctiva  bulbi  and  the  connection  between  these  two  parts  which  is  called 
Conjunctiva  Fornix.  The  conjunctiva  tarsi  is  best  seen  by  everting  the  lid. 
It  has  a  smooth  surface  and  is  intimately  and  immovably  adherent  to  the  lids. 

The  Region  of  Transition  (or  fornix)  is  very  readily  brought  to  view 
in  the  lower  lid  by  drawing  the  lid  down  while  the  eye  looks  up.  In  the 
upper  lid,  on  the  contrary,  the  fold  of  transition  is  hard  to  see,  unless  we 
make  a  double  eversion  of  the  lid.  The  region  of  transition  is  the  loosest 
part  of  the  conjunctiva,  being  so  abundant  that  it  lies  in  horizontal  folds. 
This  arrangement  insures  the  eye  its  free  power  of  movement.  If  the 
conjunctiva  were  to  pass  directly  from  the  lid  to  the  eye,  as  is  sometimes 
observed  in  consequence  of  disease  of  the  conjunctiva,  every  movement  of 
the  eyeball  would  be  transmitted  to  the  lids ;  and  if  one  of  the  lids  were  held 
still  with  the  finger,  the  eyeball  would  be  hampered  by  it  in  its  movements. 
The  conjunctiva  is  present  in  such  quantity  at  the  fornix  that  the  eye 
is  able  to  move  in  complete  independence  of  the  lids,  the  folds  in  the  region 
of  transition  being  smoothed  out  or  crumpled  together,  as  the  movement  may 
require.  Appearing  through  the  lower  fold  of  transition  are  the  extensive  sub- 
jacent plexus  of  veins  and  also  the  white  glistening  fascia.  Its  lax  char- 
acter and  also  its  abundant  blood  supply  render  the  fold  of  transition  par- 
ticularly liable  to  great  swelling  in  inflammations  of  the  conjunctiva. 

The  microscope  shows  that  the  palpebral  conjunctiva  is  covered  with 
a  laminated  cylindrical  epithelium.  The  mucous  membrane  proper  is  of 
adenoid  character — that  is,  even  in  the  healthy  state  it  contains  an  abundant 
quantity  of  round  cells  (lymph  corpuscles),  which  notably  increase  in  num- 
ber with  every  inflammation  of  the  conjunctiva.  Of  glands  it  possesses 
acinous  mucous  glands,  which  are  found  along  the  convex  border  of  the 
tarsus.     Analogous  glands  are  present  in  the  fornix  conjunctiva. 


"The  conjunctiva  of  the  upper  lid  obtains  its  blood  supply  from  two 
arterial  arches,  the  arcus  tarseus  superior  and  the  arcus  tarseus  inferior. 
These  lie  upon  the  anterior  surface  of  the  tarsus,  near  its  upper  and  its 
lower  edges.  To  reach  the  conjunctiva,  the  branches  of  the  arcus  tarseus 
inferior  perforate  the  tarsus  through  its  entire  thickness  from  before  back- 
ward, two  or  three  m.m  above  the  free  edge  of  the  lid.  The  line  along 
which  the  vessels  come  out  from  the  tarsus  is  marked  by  a  shallow  furrow 
(sulcus  subtarsalis)  on  the  conjunctival  surface  of  the  lid.  On  the  lower 
lid  there  is  only  one  arterial  arch. 

The  Conjunctiva  Bulbi  covers  the  anterior  surface  of  the  eyeball. 
It  has  no  aperture  corresponding  to  the  cornea,  but  continues,  even  if  with 
altered  character,  over  the  latter.  This  continuity  of  the  conjunctiva  makes 
it  plain  to  us  why  morbid  processes  of  the  latter  do  not  stop  at  the  margin 
of  the  cornea  but  are  continued  upon  its  surface,  as  we  see  very 
clearly  in  trachoma  and  in  conjunctivitis  eczematosa.  The  two  divisions 
of  the  conjunctiva  bulbi  are  distinguished  as  the  conjunctiva  scleraa  and 
conjuctiva  cornea. 

The  Conjunctiva  Cornea  is  perfectly  transparent,  and  is  so  intimately 
adherent  to  the  cornea  proper  that  it  must  be  regarded  as  the  uppermost 
layer  of  the  latter,  and  is  better  treated  in  connection  with  the  cornea  itself. 

The  Conjunctiva  Sclerae  covers  the  anterior  segment  of  the  sclera 
in  the  form  of  a  thin  pellicle.  It  is  connected  with  the  sclera  by  lax  con- 
nective tissue  (the  episcleral  tissue)  so  loosely  that  it  can  readily  be  moved 
about  from  side  to  side  upon  the  sclera.  It  is  only  at  the  periphery  of  the 
cornea,  where  it  ends  in  a  sharp  edge,  the  limbus  (Limbus-hem)  conjunctiva, 
that  the  conjunctiva  sclerae  is  intimately  adherent  to  its  substratum.  It  is 
very  thin  and  elastic  and  lets  the  white  sclera  be  seen  through  it  plainly,  thus 
forming  the  **white  of  the  eye." 

In  old  people  there  is  a  spot  at  the  inner  and  the  outer  margins  of 
the  cornea  which  contrasts  by  its  yellow  color  with  this  whiteness.  This 
has  the  shape  of  a  triangle  with  its  base  at  the  corneal  margin,  and  projects 
a  little  above  the  rest  of  the  conjunctiva.  It  is  called  the  interpalpebral 
spot  or  the  Pinguecula,  and  is  caused  by  an  alteration,  that  has  taken  place 
in  the  tissues  of  that  part  of  the  conjunctiva  which,  being  included  in  the 
interpalpebral  fissure,  is  constantly  exposed  to  atmospheric  influences. 

The  conjunctiva  sclerae  is  covered  with  laminated  pavement  epithe- 
lium and  contains  no  glands.  At  the  inner  angle  of  the  eye  it  forms  a 
crescentic  duplication,  the  semilunar  fold  (plica  semilunaris)  ^  which  repre- 
sents an  abortive  remnant  of  the  palpebra  tertia  of  animals.  To  the  inside 
of  the  semilunar  fold  is  a  small,  reddish,  nipple-like  prominence,  the  car- 
uncle, which  occupies  the  bottom  of  the  horseshoe-shaped  excavation  at  the 
angle  of  the  eye.  This  is  shown  to  be  histologically  a  small  island  made  of 
skin,  containing  sebaceous  and  sweat  glands  and  having  its  surface  covered 
with  delicate,  light-colored  hairs. 

The  conjunctiva  of  the  eyeball  receives  its  blood  vessels  chiefly  from 
the  vessels  of  the  fold  of  transition — the  posterior  conjunctival  vessels. 
Furthermore,  the  anterior  ciliary  vessels  take  part  in  supplying  the  con- 
junctiva with  blood.  These  vessels  come  from  the  four  recti  muscles  and 
run  under  the  conjunctiva  (through  which  they  are  visible,  shining  with  a 

8 


bluish  luster)  until  near  the  edge  of  the  cornea,  where  they  suddenly  dis- 
appear, since  they  pass  through  the  sclera  into  the  interior  of  the  eye.  But, 
before  this  happens,  they  give  off  branches  which  end  in  vascular  loops, 
in  the  limbus  conjunctiva  directly  at  the  margin  of  the  cornea  (marginal 
network  of  the  cornea).  This  latter  is  of  great  importance  for  the  cornea, 
which  is  chiefly  dependent  upon  it  for  nutrition.  Other  branches  of 
the  ciliary  vessels  (anterior  conjunctival  vessels)  run  backward  in  the 
•conjunctiva  toward  the  posterior  conjunctival  vessels  and  anastomose  with 
them. 

We  have,  therefore,  in  the  conjunctiva  two  vascular  systems — ^that  of 
the  posterior  conjunctival  vessels  and  that  of  the  anterior  ciliary  vessels. 
According  as  the  one  or  the  other  system  is  over  distended  with  blood, 
the  conjunctiva  has  a  different  aspect,  which  we  designate  respectively  as 
conjunctival  and  as  ciliary  injection. 


The  Lachrymal  Apparatus. 

The  Lachrymal  Apparatus  consists  of  the  lachrymal  gland  and  the 
lachrymal  passages. 

The  Lachrymal  Gland  (glandula  lacrimalis)  is  an  acinous  gland  which 
consists  of  two  divisions.  The  larger  of  these,  known  as  the  superior 
lachrymal  gland,  lies  in  the  upper  external  angle  of  the  orbit  in  a  depres- 
sion in  the  bony  wall  of  the  latter,  the  fossa  glandulae  lacrimalis.  The 
excretory  ducts  of  the  superior  lachrymal  gland  pass  downward  and  empty 
into  the  external  half  of  the  superior  fornix  conjunctiva. 

The  second  division  of  the  lachrymal  gland — the  inferior  lachrymal 
gland — is  much  smaller,  and  consists  only  of  one  or  two  lobules,  for  which 
reason  it  is  also  known  as  the  accessory  lachrymal  gland.  Its  lobules  lie 
along  the  excretory  ducts  of  the  superior  gland  directly  beneath  the  mucous 
membrane  of  the  fornix.  If  the  upper  lid  is  everted  and  at  the  same  time 
the  eye  is  made  to  look  downward,  we  often  see  the  conjunctiva  of  the 
fornix  in  the  vicinity  of  the  outer  angle  of  the  lid  pushed  forward  by  a 
soft  mass  which  is  in  fact  the  inferior  lachrymal  gland.  Krause's  glands 
form  a  sort  of  continuation  of  the  lobules  of  the  inferior  lachrymal  gland 
over  the  fornix  as  far  as  its  inner  end.  Their  structure  is  that  of  the 
lachrymal  gland,  so  that  they  may  be  regarded  as  the  ultimate  scattered 
outlying  portions  of  the  latter. 

The  Lachrymal  Passages  begin  with  the  puncta  lacrimalia.  These 
lie  on  the  free  border  of  the  upper  and  lower  lid  (upper  and  lower  punctum) . 
In  its  downward  course  the  lachrymal  duct  deviates  a  little  backward 
and  outward  from  the  vertical.  Hence  the  two  lachrymal  channels  diverge 
as  they  go  down,  the  lachrymal  sacs  being  less  far  apart  than  are  the  lower 
orifices  of  the  lachrymal  ducts.  We  can  represent  the  course  of  the  lachry- 
mal channel  on  the  living  subject  by  placing  a  straight  sound  in  such  a  way 
as  to  lie  at  its  upper  part  upon  the  middle  of  the  internal  palpebral  liga- 
ment, and  below  upon  the  furrow  forming  the  boundary  line  between  the 
cheek  and  the  alae  of  the  nose.  The  position  of  this  sound  gives  precisely  the 
direction  of  the  lachrjmial  duct.     If  we  place  a  sound  in  this  way  on  each 

9 


side  of  the  nose,  we  see  how  the  sounds  diverge  as  they  go  down,  and  we  can 
readily  convince  ourselves  that  the  degree  of  divergence  differs  in  different 
individuals.  The  divergence,  in  fact,  depends  upon  the  breadth  of  the  root 
of  the  nose  on  the  one  hand,  and  upon  the  breadth  of  the  inferior  nasal 
orifice  on  the  other.  These  facts  are  of  importance  with  regard  to  the 
operation  of  sounding  the  lachrymal  duct,  in  the  performance  of  which  the 
sound  must  be  pushed  along  in  the  direction  of  the  duct. 

The  mucous  membrane  of  the  lachrymal  sac  and  that  of  the  lachrymal 
duct  form  one  continuous  whole.  There  is,  therefore,  no  sharp  dividing 
line  between  these  two  structures.  They  are  mainly  distinguished  by  the 
fact  that  the  lachrymal  sac  lies  against  the  bone  {the  lachrymal  bone)  at 
one  side  only,  and  everywhere  else  is  free,  while  the  lachrymal  duct  is 
inclosed  on  all  sides  by  boney  walls.  It  follows  from  this  that,  in  engorge- 
ment of  the  lachrymal  channels  with  fluid,  it  is  only  the  lachrymal  sac 
which  is  distended  so  as  to  appear  as  a  visible  swelling  at  the  inner  angle 
of  the  eye.  The  lachrymal  duct  cannot  be  distended;  on  the  contrary,  it 
is  the  favorite  seat  of  constrictions,  which  again  do  not  occur  in  the  lachry- 
mal sac.  The  formation  of  these  constrictions  is  facilitated  by  the  fact  that 
a  dense  plexus  of  wide  veins,  analogous  to  the  venous  plexuses  lie  beneath 
and  near  the  inner  extremity  of  the  lid  at  the  spot  where  the  tarsus  ter- 
minates. They  are  situated  upon  small  elevations,  the  lachrymal  papillas 
{papillae  lacHmales) ,  and  form  the  orifices  of  the  canaliculi  lacrimales. 
These  latter,  starting  from  the  puncta,  run  at  first  vertically  for  a  short 
distance — i.  e.,  in  the  upper  lid  run  upward  and  in  the  lower  lid  down- 
ward; then  they  bend  at  a  right  angle  and  become  directed  toward  the 
lachrymal  sac.  In  so  doing  they  first  pass  behind  the  caruncle,  and  con- 
verging more  and  more,  at  length  reach  the  lachrymal  sac.  Into  this  they 
empty,  either  separately  or  after  having  united  to  form  a  short  common 
trunk. 

The  Lachrymal  Sac  {sa^cus  lacrimalis)  lies  in  the  inner  angle  of  the 
eye  in  the  cleft  {fossa  sacci  lacrimalis)  which  the  lachrymal  bone  forms  for 
its  reception.  The  lachrymal  bone  bounds  the  lachrymal  sac  on  the  inside, 
while  to  the  front  and  outside  it  is  inclosed  by  the  two  branches  of  the  liga- 
mentum  palpebrale  mediale.  This  relation  of  the  lachrymal  sac  to  the  in- 
ternal palpebral  ligament  enables  us  to  determine  the  position  of  the  former 
— a  matter  which  is  of  importance  when  operations  are  concerned.  If  by 
drawing  the  lids  outward  we  put  them  on  the  stretch  and  so  cause  the  pal- 
pebral ligament  to  project,  it  can  be  seen  that  the  lachrymal  sac  lies  behind 
the  latter,  and  in  such  a  way  as  to  rise  just  above  it  by  its  summit  or  fundus. 

At  the  spot  where  the  cleft  of  the  lachrymal  bone  merges  into  the 
bony  canal  the  lachrymal  sac  passes  into  the  nasal  or  Lachrymal  Duct 
{ductus  hbcrimalis) .  The  point  where  this  transition  occurs  constitutes  the 
narrowest  part  of  the  whole  lachrymal  channel,  and  is,  therefore,  particu- 
larly liable  to  the  formation  of  pathological  contractions  {strictures) .  From 
this  point  the  lachrymal  duct  passes  downward  and  empties  into  the  nasal 
fossa  below  the  inferior  turbinated  body  beneath  the  mucous  membrane  of 
the  turbinated  bodies  and  is  interposed  between  the  mucous  membrane  of  the 
lachrymal  duct  and  the  bony  wall.  The  swelling  of  these  veins  is  alone  suf- 
ficient to  contract  or  to  close  entirely  the  lumen  of  the  duct. 

10 


The  lachrymal  passages  are  always  filled  with  a  small  quantity  of 
lachrjnnal  fluid.  If  air  is  found  in  them,  it  is  to  be  regarded  as  a  patho- 
logical condition. 

The  Lagjirymal  Secretion  contains  only  a  few  solid  constituents, 
the  main  part  of  which  is  sodium  chloride  {hence  "salty"  tears).  In  the 
normal  state  the  lachrymal  gland  secretes  scarcely  any  more  liquid  than 
is  lost  by  evaporation  from  the  surface  of  the  eyeball,  so  that  but  very  small 
quantities  of  fluid  are  discharged  into  the  nose.  It  is  only  when  the  secre- 
tion is  increased,  either  in  consequence  of  physical  stimulation  or  of  irrita- 
tion of  the  eye,  that  any  considerable  quantity  of  tears  is  discharged  into 
the  nose,  where  its  presence  is  manifested  by  repeated  blowing  of  the  nose. 

The  moistening  of  the  eyeball  is  not  due  to  the  lachrymal  glands  alone. 
The  secretion  of  the  conjunctiva  itself,  and  also  of  its  mucous  glands,  par- 
ticipate in  the  performance  of  this  act.  Hence  it  follows  that  even  after 
removal  or  degeneration  of  the  lachrymal  gland  the  eye  does  not  become  dry. 

In  the  Conduction  of  Tears  into  the  nose  there  are  two  factors  to  be 
considered :  the  entrance  of  the  tears  into  the  lachrymal  sac ;  and  their  trans- 
mission from  the  latter  to  the  nose. 

(a)  The  conveyance  of  tears  through  the  puncta  into  the  lachrymal 
sac  is  effected  by  the  act  of  winking.  This  takes  place  in  such  a  way  that 
the  palpebral  fissure  is  closed  by  a  movement  beginning  at  the  outer  and 
extending  to  the  inner  angle  of  the  eye.  Since  the  tears  cannot  flow  off  over 
the  border  of  the  lid  owing  to  the  way  in  which  the  latter  is  lubricated 
with  fatty  matter,  they  accumulate  in  the  horseshoe-shaped  notch  in  the 
inner  angle  of  the  eye,  and  form  the  lacus  lacrimalis  into  which  the  punctum 
dips.  If  there  is  a  perfect  and  water  tight  closure  of  the  lids,  the  pressure 
exerted  by  the  latter  at  length  forces  the  tears  into  the  puncta.  The  pas- 
sage of  the  tears  into  the  lachrymal  sac  is  facilitated  by  the  passive  dilata- 
tion of  the  latter  that  occurs  as  the  lid  is  closing;  for  the  fibers  of  the 
palpebral  portion  of  the  orbicularis  arise  in  part  from  the  internal  palpebral 
ligament,  and  hence,  in  contracting  as  they  do  during  the  closure  of  the  lids, 
draw  the  ligament  away  from  the  lachrymal  bone.  The  anterior  wall  of 
the  lachrymal  sac  being  connected  with  the  palpebral  ligament,  is  drawn  up 
at  the  same  tim.e  with  it,  so  that  the  lachrymal  sac  is  dilated  and  the  con- 
tents of  the  canaliculi  are,  so  to  speak,  sucked  into  the  sac. 

(b)  The  conveyance  of  the  tears  from  the  lachrymal  sac  into  the  nose 
is  due  partly  to  the  constant  entrance  of  fresh  charges  of  tears  from  the 
canaliculi  and  partly  to  the  weight  of  the  fluid;  but  the  chief  part  in  the 
process  is  performed  by  the  elasticity  of  the  lachrymal  sac.  In  virtue  of  this 
elasticity  the  sac  when  distended  by  the  tears  tends  to  contract  again  and 
thus  expel  the  tears.  Hence,  in  those  pathological  cases  in  which  the  lachry- 
mal sac  has  lost  its  elasticity  (atony  of  the  sac)  we  observe  that  the  con- 
duction of  tears  downward  is  arrested,  even  though  the  nasal  duct  is 
completely  pervious. 

The  mucous  membrane  of  the  canaliculi  is  lined  with  laminated  pave- 
ment epithelium;  that  of  the  lachrymal  sac  and  nasal  duct  with  a  single 
layer  of  cylindrical  epithelium.  Acinous  mucous  glands  are  frequently 
found  in  the  latter.  The  mucous  membrane  at  different  spots  projects  in 
the  form  of  folds  into  the  lumen  of  the  lachrymal  passages,  a  phenomenon 
which  has  been  described  as  a  formation  of  valves. 

11 


The  largest  of  these  folds  is  Hasner's  valve,  at  the  lower  orifice  of  the 
nasal  duct.  This,  however,  is  not  a  true  valve,  any  more  than  are  the  others 
— that  is,  not  a  valve  which  could  close  up  the  lumen  of  the  lachrymal 
channel.  On  the  contrary,  it  is  simply  a  fold  produced  by  the  great  obliquity 
with  which  the  nasal  duct  passes  through  the  mucous  membrane  of  the  nasal 
fossa.  Like  the  other  folds  of  mucous  membrane  in  the  lachrymal  passages, 
it  is  not  of  constant  occurrence. 

Duplication  of  the  puncta  and  canaliculi,  and  also  their  absence,  have 
been  recorded  as  congenital  anomalies. 

Different  theories  have  been  put  forth  to  account  for  the  transmis- 
sion of  tears  into  the  nose.  It  is  certain  that  perfect  closure  of  the  lids 
forms  an  indispensable  condition  for  the  conduction  of  tears.  If  this  closure 
is  interfered  with — e.  g.,  by  paralysis  of  the  orbicularis,  narrowing  of  the 
lids,  notching  of  the  border  of  the  lids,  etc. — epiphora  at  once  makes  its 
appearance.  The  passage  of  tears  into  the  lachrymal  sac  takes  place  even 
when  the  rest  of  the  way  to  the  nose  is  cut  off  by  the  obliteration  of  the 
nasal  duct.  Hence  it  follows  that  the  tears  are  not  sucked  into  the  lachry- 
mal sac  through  the  rarefaction  of  the  air  in  the  nose  during  inspiration. 

THE  EYEBALL. 

The  Eyeball  is  situated  in  the  anterior  part  of  the  orbit,  to  its  outer 
side  and  rests  upon  a  cushion  of  cellular  tissue  and  fat.  It  is  protected 
in  front  by  the  eyelids.  In  form  it  is  a  sphere  of  about  25  mm.  in  diameter 
having  the  segment  of  a  smaller  sphere  engrafted  upon  its  anterior  surface, 
which  slightly  increases  its  anterior  posterior  diameter.  The  axes  of  the 
two  balls  are  parallel  with  each  other,  but  do  not  correspond  with  the  axis 
of  the  orbits,  which  are  directed  outward.  The  eyeball  does  not  reach  its 
full  size  until  about  20  years  of  age.  Its  weight  is  about  95  grains.  The 
eyeball  is  composed  of  tunics  or  coats  and  of  refracting  media  called  hu- 
mours. The  tunics  are  3  in  number.  Named  from  without  inward  they 
are  first  sclerotica  and  cornea;  second  choroid,  iris  and  ciliary  processes; 
third  retina  and  zonula  cilaris. 

The  First  Tunic.  The  Sclerotic  and  Cornea  are  the  outside  coats 
of  the  ball.  Four-fifths  of  the  ball  is  enveloped  by  the  sclerotic ;  the  remain- 
ing one-fifth  by  the  cornea. 

The  Sclerotic,  from  a  Greek  word  meaning  hard,  is  a  tough  white 
fibrous  tunic  or  coat,  and  serves  to  give  shape  to  the  eye  and  protect  its  more 
delicate  interior.  The  Sclerotic  is  thickest  at  its  posterior  portion  and 
gradually  becomes  thinner  as  it  approaches  the  cornea,  where  it  again 
thickens  *at  the  sclero  corneal  junction.  Entirely  within  the  sclera  at  the 
sclero  corneal  junction  is  the  circular  venous  sinus  (canal  of  Schlemm), 
which  is  the  outlet,  whereby  the  aqueous  humor  finds  its  way  into  the  cir- 
culation. 

The  sclero  corneal  junction  is  beveled  and  receives  the  cornea  in  the 
same  way  as  the  crystal  of  a  watch  is  received  by  the  groove  in  its  case. 

The  Cornea  {comu,  a  horn)  is  the  transparent  part  of  the  external 
coat  of  the  eyeball  and  projects  beyond  the  curving  surface  of  the  sclerotic 
and  is,  therefore,  the  segment  of  a  smaller  sphere.     In  form  it  is  slightly 

12 


elliptical,  the  vertical  diameter  measuring  11  mm.,  while  the  horizontal 
diameter  measures  12  mm.  The  curvature  of  the  posterior  surface  is  some- 
what greater  than  the  anterior  surface,  the  difference  in  the  curvature 
resulting  in  an  increase  in  the  thickness  of  the  edges,  the  edges  measuring 
about  1  mm.  A  vertical  section  of  the  cornea  shows  it  to  consist  of  five 
layers : 

1.  The  Anterior  Epithelium. 

2.  Bowman's  Membrane  (Anterior  elastic  Lamina). 

3.  The  Stroma  (Substantia  propria). 

4.  Descement's  Membrane  (Posterior  elastic  Lamina). 

5.  The  Posterior  Epithelium. 

1.  The  Anterior  Epithelium  is  a  continuation  of  the  conjunctiva 
and  consists  of  pavement  epithelium  made  up  of  cylindrical  round  and  flat 
cells.     It  is  thickest  at  the  periphery  and  thinnest  at  the  center  of  the  cornea. 

2.  Bowman's  Membrane  is  a  delicate,  thin,  homogeneous  membrane 
firmly  attached  to  the  lamellae  of  the  stroma.  In  pathological  conditions 
and  after  death,  the  anterior  epithelium  separates  very  rapidly  from  Bow- 
man's membrane. 

3.  The  Stroma  or  true  cornea  forming  95%  of  the  whole,  is  com- 
posed of  a  ground  substance  and  of  cells.  The  ground  substance  is  com- 
posed of  fine  fibrillas  of  connective  tissue,  united  together  by  a  cement  sub- 
stance into  flat  bundles  arranged  in  nearly  regular  lamellas.  Between  these 
lamella  are  open  spaces  which  are  filled  with  lymph  and  are  called  lymph 
spaces;  these  are  connected  with  one  another  by  small  canals.  Through 
these  spaces  and  canals  circulate  the  Ijnnph  upon  which  the  cornea  is  de- 
pendent for  its  nourishment.  Circulating  also  in  these  canals  and  spaces 
are  a  few  white  blood  corpuscles.  In  the  normal  eye  they  are  very  few  in 
number,  but  as  soon  as  any  pathological  condition  occurs  these  white  blood 
corpuscles  escape  from  the  net  work  of  blood  vessels  at  the  margin  of  the 
cornea  and  increase  very  rapidly  and  play  a  very  important  part  in  any 
inflammation  of  the  cornea. 

4.  Descement's  Membrane  is  a  thin  homogeneous  hyaloid  membrane 
which  serves  to  give  the  cornea  its  correct  curvature.  It  is  very  resistant 
to  all  pathological  changes  and  when  the  entire  cornea  is  broken  down  into 
pus,  this  membrane  will  remain  unimpaired  for  days. 

5.  The  Posterior  Epithelium  is  a  single  layer  of  flattened  cells  lining 
the  posterior  surface  of  Descement's  membrane. 

The  Corneal  Nerves  arise  partly  from  the  terminal  branches  of  the 
long  ciliary  and  partly  from  the  nerves  of  the  bulbar  conjunctiva.  They 
are  abundantly  distributed  to  the  upper-most  layers  of  the  stroma  directly 
beneath  Bov/man's  membrane.  Owing  to  the  large  nerve  supply  lesions  of 
the  cornea  are  particularly  painful. 

The  Second  Tunic  is  composed  of  the  Choroid,  Ciliary  Body  and  Iris. 
It  lines  the  inner  side  of  the  sclerotic,  is  perforated  posteriorly  by  the  optic 
nerve,  and  in  front  has  a  circular  opening  called  the  pupil. 

The  Choroid  is  a  vascular  membrane  of  a  rich  chocolate  brown  color 
upon  its   internal   surface  and  a   deep  black  within.     It  lies  between  the 

13 


sclerotic  and  retina  and  extends  from  the  optic  nerve  to  the  ciliary  body. 
It  is  pierced  posteriorly  for  the  passage  of  the  optic  nerve  and  is  connected 
anteriorly  with  the  iris,  ciliary  processes  and  with  the  line  of  junction  of 
the  cornea  anu  sclerotic,  by  a  dense  white  structure,  the  ciliary  ligament, 
which  surrounds  the  circumference  of  the  iris  like  a  ring.  The  choroid 
membrane  is  composed  of  three  layers:  external,  middle  and  internal.  The 
external  layer  is  composed  largely  of  veins  and  has  been  named  Venae  Vor- 
tecosoe.  The  middle  is  composed  largely  of  minute  arteries.  The  internal 
layer  (or  membrana  pigments)  as  its  name  implies  is  composed  of  pigment 
cells.     The  presence  of  these  cells  give  the  black  appearance  to  the  choroid. 

The  Ciliary  Body  is  composed  of  the  Ciliary  Muscle  and  the  Ciliary 
Process  and  is  that  portion  lying  between  the  choroid  and  iris.  It  is  con- 
tinuous with  both,  and  is  about  ^  of  an  inch  in  width. 

The  Ciliary  Muscle  is  composed  of  circular  and  radiating  fibres.  The 
circular  fibers  occupy  that  portion  of  the  ciliary  body  near  its  junction  with 
the  iris,  while  the  radiating  fibers  are  confined  to  the  outer  portion  of  the 
ciliary  body  arising  tendinously  from  the  sclero-corneal  junction,  the  poster- 
ior elastic  lamina,  and  from  the  ligamentum  pectinatum  iridis,  and  are 
directed  backward  and  inward  where  they  terminate  in  the  choroid. 

The  Ciliary  Process  is  formed  by  folds  (processes)  of  the  choroid 
at  its  interior  margin.  The  folds  or  processes  form  a  circle  behind  the  lens 
and  cover  the  inner  surface  of  the  ciliary  body.  They  are  deepest  and 
thickest  at  their  forepart  and  gradually  taper  into  the  choroid.  Within  the 
folds  of  the  ciliary  processes  are  received  the  corresponding  folds  of  the 
suspensory  ligament,  which  is  also  called  Zonula  Zinnii. 

The  Iris  is  a  disk-shaped  membrane  of  about  one-half  inch  in  diameter 
(12  m) ,  perforated  in  the  center,  which  perforation  is  called  the  pupil.  By 
its  peripheral  or  ciliary  border  it  is  continuous  with  the  ciliary  body  and  is 
held  in  position  by  the  pectonate  ligament  and  also  by  the  anterior  capsule  of 
the  crystaline  lens  upon  which  the  pupillary  border  rests. 

The  anterior  surface  of  the  iris  is  lined  with  a  continuation  of  the 
posterior  epithelium  of  the  cornea,  and  if  examined  closely  is  found  to  be 
composed  of  elevations  and  depressions,  which  are  caused  by  the  blood  vessels 
lying  in  the  stroma  of  the  iris  and  running  from  the  ciliary  to  the  pupillary 
margin.  Near  the  pupillary  margin  these  vessels  interlace  with  a  ring  of 
circular  ridges.  This  divides  the  iris  into  two  zones:  the  ciliary  zone  and 
the  pupillary  zone. 

The  posterior  surface  of  the  iris  is  covered  by  the  posterior  lining 
membrane  and  the  retinal  pigment  layer,  which  layer  gives  the  different 
colors  to  the  iris.  The  color  of  the  irides  in  the  same  individual  are,  as  a  rule 
similar,  but  there  are  instances  of  congenital  differences  in  color  in  perfectly 
normal  irides. 

The  iris,  by  its  ability  to  open  and  close,  makes  the  pupil  larger  or 
smaller  and  regulates  the  amount  of  light  which  enters  the  eye.  Ordinarily 
the  pupil  is  about  one-eighth  to  one-fifth  of  an  inch  (3  to  5  mm.)  in  dia- 
meter. It  should  be  round  and  promptly  react  to  light  stimulation.  Under 
the  same  illumination  both  pupils  should  be  uniform  in  size.  If  one  eye 
should  be  shaded  the  pupil  of  the  shaded  eye  should  act  in  harmony  with 
the  one  exposed  to  light.     This  is  termed  the  consenual  action  of  the  pupils. 

14 


If  the  iris  be  thin  and  the  rays  of  light  pass  through  its  substance,  they 
are  immediately  absorbed  by  the  uvea,  and  if  that  layer  be  insufficient,  they 
are  taken  up  by  the  black  pigment  of  the  ciliary  process.  In  Albinos, 
where  there  is  an  absence  of  the  black  pigment,  the  rays  of  light  traverse  or 
pass  through  the  iris,  and  even  the  sclerotic  and  so  overwhelm  the  eye  with 
light,  that  the  sight  is  very  much  diminished,  except  in  the  dimness  of  even- 
ing. In  the  manufacture  of  optical  instruments,  care  is  taken  to  color  their 
interior  black  with  the  same  object,  the  absorption  of  all  extraneous  rays  of 
light. 

The  Third  Tunic,  or  inner  tunis  of  the  eye,  is  the  Retina,  a  thin, 
delicate  membrane  formed  from  the  fibers  of  the  optic  nerve.  In  the  living 
eye  it  is  perfectly  transparent  and  of  a  purplish  red  color.  The  retina  is 
composed  of  two  kinds  of  tissue,  the  nervous  tissue  and  the  supporting  tissue. 
The  function  of  the  latter  is  to  maintain  and  support  the  extremely  delicate 
nervous  tissue.  The  retina  is  attached  at  the  entrance  of  the  optic  nerve 
and  at  the  ora  serrata.  It  is  not  attached  to  the  choroid,  but  simply  lies  on  it. 

An  examination  of  the  normal  eye  with  the  ophthalmoscope  reveals  the 
optic  disk,  the  retinal  vessels  and  the  macula  lutea,  which  is  the  thinnest 
part  of  the  retina  and  the  part  that  has  the  most  delicate  perception;  this 
is  also  called  the  fovea  centralis,  yellow  spot.  When  we  wish  to  get  a  precise 
perception  of  an  object  we  so  adjust  our  eye  that  the  image  shall  fall  upon 
this  point.     We  fix  the  object  or  bring  it  within  our  central  vision. 

In  contradistinction  to  central  vision,  we  have  peripheral  or  indirect 
vision,  which  is  vision  with  all  the  balance  of  the  retina.  The  farther  from 
the  fovea  centralis  the  image  is  produced  upon  the  retina,  the  less  distinct 
is  the  perception  of  its  shape.  The  images  falling  upon  the  periphery  of 
the  retina  give  us  warning  signals,  which  make  us  cast  our  eye  directly  upon 
the  objects  which  excite  the  image.  Peripheral  vision  is,  therefore,  very  im- 
portant in  walking. 

The  microscope  shows  the  retina  to  be  composed  of  ten  different  layers. 
They  are  as  follows: 

1.  Memhrana  limitans  interna. 

2.  Fibrous  layer. 

3.  Vesicular  layer. 

4.  Inner  molecular  layer. 

5.  Inner  nuclear  layer. 

6.  Outer  molecular  layer. 

7.  Outer  nuclear  layer. 

8.  External  limiting  membrane. 

9.  Jacob's  membrane  or  rods  and  cones. 
10.     Pigmentary  layer. 

Jacob's  membrane  or  rods  and  cones  is  the  most  important  part  of  the 
retina,  as  it  is  connected  by  nerve-fibrils  with  the  layer  of  nerve  fibers  which 
convey  the  visual  impressions  through  the  optic  nerve  to  the  brain.  The  optic 
nerve  collects  its  fibers  from  the  retina.  It  pierces  the  choroid  and  sclera 
and  comes  out  a  little  to  the  nasal  side  of  the  posterior  pole  of  the  eye.  This 
portion  of  the  nerve  is  called  the  intra  ocular,  while  the  part  extending 
from  the  back  of  the  ball  to  the  optic  foramen  is  called  the  orbital,  and  that 
part  extending  from  the  foramen  to  the  optic  chiasm  is  called  the  intra 

15 


cranial,  so  that  we  have  the  optic  nerve  divided  into  three  portions:     THE 

INTRA  OCULAR,   THE  ORBITAL  and  THE   INTRA    CRANIA.       It    is    about    one-sixth 

of  an  inch  in  diameter  (-4  mm.)  and  is  supplied  by  blood  vessels  from  the 
ophthalmic  artery.  The  optic  disk  is  that  portion  of  the  optic  nerve  included 
within  the  margins  of  the  choroidal  foramen.  It  is  usually  round  in  shape 
and  is  pierced  in  the  center  by  the  central  retinal  artery  and  vein.  Both 
the  artery  and  vein  bifurcate,  one  branch  of  each  passing  vertically  upward 
and  the  other  downward  to  the  retina.  By  the  aid  of  the  ophthalmoscope 
the  vein  can  easily  be  recognized  from  the  artery  by  its  size  and  color. 

The  Humors  of  the  Eye  Ball.  The  Aqueous  Humor  is  a  clear, 
colorless,  transparent  liquid  filling  the  anterior  and  posterior  chambers  of  the 
ball.  It  is  faintly  alkaline  and  has  a  sp.  gr.  of  about  1005.  It  is  secreted  by 
the  ciliary  processes  and  the  posterior  surface  of  the  iris.  If  the  anterior 
and  posterior  chamber  should  be  evacuated  by  accident  or  by  an  operation 
the  aqueous  humor  is  rapidly  reproduced. 

The  Vitreous  Humor  is  a  transparent,  colorless,  gelatinous  mass  which 
fills  the  vitreous  cavity.  It  is  surrounded  by  a  transparent  capsule  called 
the  hyaloid  membrane.  The  anterior  portion  of  the  vitreous  humor  has  a 
deep  depression  called  the  hyaloid  fossa.  It  is  within  this  fossa  that  the 
posterior  portion  of  the  crystalline  lens  rests.  Beginning  at  the  papilla  of 
the  optic  nerve  and  extending  to  the  posterior  surface  of  the  crystalline  lens 
is  a  canal  (canalis  hyaloideus).  During  foetal  life  the  hyaloid  artery  runs 
into  this  canal.  In  the  fully  developed  eye  it  probably  serves  as  a  lymph 
channel. 

The  principal  function  of  the  vitreous  humor  is  to  hold  the  retina  in 
place. 

The  Crystalline  Lens  lies  within  the  circle  formed  by  the  ciliary 
processes,  and  is  kept  in  position  by  the  suspensory  ligament  or  zonula 
ciHaris.  In  this  position  it  divides  the  eye  into  a  smaller  anterior  and  a 
larger  posterior  section.  It  is  a  transparent  and  colorless  structure  of  len- 
ticular shape,  the  anterior  surface  of  which  is  less,  the  posterior  surface 
more  curved.  It  causes  the  rays  of  light  to  focus,  in  the  normal  eye,  upon 
the  retina. 

The  function  of  the  lens  is  to  enable  us  to  see  distinctly  all  objects  within 
the  range  of  vision,  no  matter  at  what  distance.  This  is  spoken  of  as  the 
power  of  accommodation.  We  can  easily  convince  ourselves  that  the  normal 
eye  possesses  such  a  power,  when  we  go  to  a  baseball  game  and  sit  back  of  the 
wire  screen.  We  cannot  see  the  screen  and  the  player  at  one  and  the  same 
time.  Another  proof  is  found  in  the  veils  worn  by  women.  This  power  of 
accommodation  is  brought  about  by  an  alteration  of  form  of  the  lens,  its  an- 
terior surface  becoming  more  convex  and  approaching  the  cornea.  This  al- 
teration in  form  is  accomplished  by  the  ciliary  muscle.  When  we  arrive  at 
a  certain  age,  which  is  about  45,  if  the  eye  was  normal  at  birth,  we  require 
spectacles  for  close  work.  This  is  due  to  the  fact  that  at  this  time  in  life 
the  ciliary  muscle  loses  some  of  its  elasticity,  and  as  a  consequence  has  not 
this  ability  to  maintain  the  effort  required  in  accommodating.  When  this 
occurs  we  designate  it  as  presbyopia,  which  means  old  sight. 

The  Ocular  Muscles  of  the  Eye  are  divided  into  the  external  extrinsic, 
and  the  internal  intrinsic. 

16 


The  External  (extrinsic)  Muscles  control  the  movement  of  the  eye 
ball  and  are  the  four  recti  (straight)  muscles  and  the  two  oblique  muscles. 
All  six  of  these  muscles  are  named  from  their  relative  positions:  superior, 
inferior,  external  and  internal. 

The  Four  Recti  Muscles  take  their  origin  from  around  the  optic 
foramen.  They  come  forward  and  are  attached,  as  their  names  indicate,  by 
means  of  short  tendons  which  spread  out  in  the  form  of  a  fan  and  become 
fused  with  the  sclera. 

The  Two  Oblique  Muscles,  the  superior  and  the  inferior,  have  a 
more  complicated  course. 

The  Superior  Oblique  arises  from  the  margin  of  the  optic  foramen  and 
runs  forward  upon  the  upper  and  inner  wall  of  the  orbit,  to  the  upper  and 
inner  margin  of  the  orbit.  It  here  passes  through  a  firm  fibrous  loop,  known 
as  the  trochlea.  From  here  it  bends  back  at  an  acute  angle  and  passes  beneath 
the  superior  rectus  to  the  eyeball.  Here  it  spreads  out  and  is  inserted  into 
the  upper  half  of  the  ball  about  in  the  vertical  meridian  and  behind  the 
equator. 

The  Inferior  Oblique  arises  from  the  orbital  plate  of  the  superior 
maxillary,  and  from  here  it  runs  upward  and  outward  and  is  inserted  about 
in  the  horizontal  meridian  and  also  behind  the  equator  of  the  ball. 

The  Nerve  Supply  of  the  External  Muscles  is  derived  from  the 
sixth  cranial  or  abducens,  the  third  cranial  or  motor  oculi,  and  the  fourth 
cranial  or  patheticus.  The  sixth  cranial  supplies  the  external  rectus.  The 
third  cranial  supplies  the  internal  rectus,  the  superior  rectus  the  inferior 
rectus  and  the  inferior  oblique.  The  fourth  cranial  supplying  the  superior 
oblique. 

The  Internal  Muscles  of  the  eye  are  the  sphincter  pupillsB  and  the 
ciliary  muscle. 

The  eye  balls  and  their  appendages  receive  their  blood  supply  from 
the  branches  of  the  ophthalmic  artery. 

The  Supra-orbital  Artery  supplies  the  superior  rectus  and  levator 
palpebraB. 

The  Superior  Palperbral  Artery  supplies  the  upper  lid. 
The  Inferior  Palpebral  Artery  supplies  the  lower  lid. 

The  Lachrymal  Artery  supplies  the  lachrymal  glands  and  the  con- 
junctiva. 

The  Nasal  Artery  supplies  the  lachrymal  sac. 

The  Superior  Muscular  Artery  contributes  to  the  levator  palpebrae, 
superior  rectus  and  superior  oblique  muscles. 

The  Inferior  Muscular  Artery  contributes  to  the  external  rectus, 
inferior  rectus  and  inferior  oblique  muscles. 

The  Infra-orbital  Artery  supplies  the  inferior  rectus  and  inferior 
oblique  muscles  (  a  branch  of  the  internf\l  maxillary  artery) . 

The  Short  Ciliary  Arteries,  about  twelve  in  number,  enter  the 
sclerotic  around  the  optic  nerve  entrance  and  supply  the  choroid  and  ciliary 
processes. 

17 


The  Long  Ciliary  Arteries  (two  in  number)  enter  the  sclerotic  on  each 
side  of  the  optic  nerve,  coming  forward  between  the  sclerotic  and  choroid 
to  the  ciliary  ligament,  where  they  divide  into  two  branches,  forming  the 
circulus  iridis,  major  and  minor. 

The  Blood  Supply  of  the  external  muscles  is  derived  from  the 
ophthalmic  artery. 

The  veins  of  the  muscles  empty  into  the  ophthalmic  and  facial  veins. 

If  we  disregard  the  conjunctiva,  there  are  no  lymphatic  vessels  in  the 
eye.  They  are  replaced  by  lymph  channels  and  lymph  spaces.  These  are 
divided  into  two  portions,  the  anterior  lymphatic  system  and  posterior 
lymphatic  system.  The  lymph  of  the  anterior  system  is  collected  into  the 
anterior  and  posterior  chambers  of  the  ball,  which  chambers  communicate  by 
means  of  the  pupil.  The  lymph  is  secreted  from  the  iris  and  ciliary  pro- 
cesses flowing  from  the  posterior  chamber  through  the  pupil  into  the  anterior 
chamber.  From  here  it  filters  through  the  sieve  work  of  the  ligamentum 
pectinatum  into  the  subjacent  canal  of  Schlemm,  and  from  here  gets  directly 
into  the  anterior  ciliary  veins. 

By  far  the  greatest  amount  of  lymph  leaves  the  eye  through  the  an- 
terior system,  and  this,  therefore,  is  more  important  than  the  posterior 
system. 

The  posterior  lymphatic  system  occupies  the  hyaloid  canal,  perichoroidal 
space  and  tenon's  space.  The  outflow  of  lymph  from  these  spaces  passes 
into  the  lymph  passages,  which  spread  out  along  the  optic  nerve.  The  pas- 
sages are  found  between  the  sheaths  of  the  optic  nerve  (the  intervaginal 
space  and  around  the  sheaths  called  the  supravaginal). 


18 


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